1. Field of the Invention
The present invention generally relates to materials testing and, in particular, to material testing machines, test piece attachments used with the material testing machines and methods for conducting material tests with the material testing machines.
2. Description of the Related Art
There have been proposed and utilized various material testing methods for obtaining values of parameters characterizing different materials. Many material tests are conducted by applying a load to a test piece. In some material testing methods, a test piece is subjected to a static or quasi-static load, and the resultant strain of the test piece is measured to obtain a stress-strain relationship for the material. In some other material testing methods, an increasing load is applied to a test piece until it fractures, so as to determine the stress acting on the test piece at the fracture. There are still other material testing methods for various purposes. In any case, it is highly important to obtain exact values of the loads actually applied to the test pieces.
Most material testing machines include a load measuring means for determining the actual load acting on the test piece. There are proposed many types of load measuring means, among which a suitable one meeting the requirements and purposes of a particular material testing machine is selected and used in that material testing machine. The requirements depend in part on the material testing methods to be conducted with the machine. Material testing methods may be categorized, in terms of the load to be applied to the test piece, into tensile test, compression test, torsion test, shearing test, bending test and others. Material testing methods may be also categorized, in terms of the strain rate of the test piece to be produced, into high-strain-rate test, moderate-strain-rate test and low-strain rate test. Those material tests which are conducted at high strain rates may be also called impact tests.
For purposes of ensuring safety of buildings and other structures against collapse, protecting passengers of automobiles safe at collision, or achieving appropriate numerical simulations of working and/or forming processes of metal parts at actual deformation rates, it becomes more and more important to determine characteristics of materials when they are subjected to deformation occurring at different strain rates in a wide strain-rate range covering from relatively low strain rates to relatively high strain rates. Accordingly, there have been strong needs for material testing machines and material testing methods, in which material tests may be conducted at different strain rates in a wide strain-rate range, and in which, in particular, those of tensile tests which necessitate relatively large deformation of the test piece and require relatively high strain rate higher than 103/sec. may be conducted with only a low level of noise found in the measured load waveform. So far, any tensile tests conducted at strain rates higher than 103/sec. have been typically subjected to a relatively high level of noise in the measured load waveform.
With the difficulties in obtaining precision measurements of material tests conducted at relatively high strain rates, measurements of material tests conducted at relatively low strain rates have been commonly used as approximations of the actually required measurements, which have been, however, often only insufficient approximations. In contrast, by utilizing the present invention, one can obtain, with ease, precision measurements of material tests conducted at different strain rates in a wide strain-rate range including relatively high strain rates corresponding to the deformation rates frequently found in an actual environment. The availability of such measurements is highly useful for many applications. For example, it may be useful in development of durable materials for structural components and automobile""s parts and components. It may be also useful in improvement of accuracy in various numerical simulations for determining the behavior of a designed structure or determining the mechanisms of forming and/or working processes which produces deformation of materials occurring at different strain rates in a wide strain-rate range.
In order to cause a test piece to produce a strain at a high strain rate, an impact load is applied to the test piece. A material testing machine including load applying means for applying an impact load to a test piece may typically also include load measuring means for measuring an impact load actually applied to the test piece.
There are proposed several load measuring methods for measuring an impact load actually applied to a test piece, among which Hopkinson bar method is commonly known and accepted. The original Hopkinson bar method has been modified in various ways into a range of variations of the Hopkinson bar method, some of which are used for conducting compression test, others are used for conducting tensile test, shearing test or other material tests.
Briefly, Hopkinson bar method uses one or two elongated bars (often called the Hopkinson bar) made of a strong and resilient material such as steel. A test piece is installed to one end surface of the single bar, or between the end surfaces of the two bars facing each other. When an impact load is applied to the test piece, a stress wave is produced at the end of the bar and propagates along the longitudinal axis of the bar toward the other end. The propagating stress wave produces a corresponding dynamic strain of the bar. Strain gages are affixed on the side surface of the bar, near the end of the bar to which the test piece is installed, in order to sense any dynamic strain of the bar. The sensed dynamic strain is used to determine the dynamic stress of the bar, which in turn is used to determine the dynamic load applied to the bar. The dynamic load applied to the bar corresponds to the dynamic load actually applied to the test piece.
From the dynamic load to the test piece thus determined, the dynamic stress of the test piece can be determined. Another means is used to determine the dynamic strain of the test piece. Then, the dynamic stress and the dynamic strain of the test piece are analyzed to determine characteristics of the material of the test piece.
The stress wave propagating from the first end (to which the test piece is installed) to the second end of the bar will be reflected by the second end to return back to the first end. The reflection would provide severe noise and disturbance to the measured load waveform if the measurement is not completed before the reflection reaches the strain gages. Thus, if one wishes to apply an impact load (or a load pulse) of relatively long duration to the test piece, the Hopkinson bar has to be long enough to provide a sufficiently long turnaround time of the stress wave propagating in the bar. Otherwise, the load measurement will be practically impossible due to the reflection of the stress wave. Indeed, for allowing use of a load pulse of significantly long duration, the bar may possibly have to be as long as ten meters or more. This leads to one of drawbacks of Hopkinson bar method that a material testing machine adopting Hopkinson bar method tends to occupy an extraordinary space. Further, for such a long bar, it is practically difficult to make precision calibration of the outputs of the strain gages with reference to the magnitude of the dynamic load (impact load) actually acting on the bar.
More recently, as an attempt to overcome the drawbacks of Hopkinson bar method described above, there has been developed another method for measuring dynamic load actually acting on a test piece, which uses, in place of a Hopkinson bar, a block of steel having a small projection. Examples of devices and methods using such a steel block are taught by Yoshitake CHUMAN, Kazuhiko KOTOH, Koichi KAIZU and Shinji TANIMURA in an article xe2x80x9cImprovement of an Apparatus for Measuring Impulsive Force Generated at a Contact Part in Collision and its Applicationxe2x80x9d, Transactions of the Japan Society of Mechanical Engineers, Vol. 59, No. 568, A, pages 139-144 (Article No. 93-0039) (December. 1993). In the article, the steel block used for measuring dynamic load is referred to as the stress sensing block.
The article describes that the stress sensing block is a steel block, which has a body with sufficient volume and mass and a small, cylindrical projection (called the xe2x80x9csensing projectionxe2x80x9d) provided on the top of the body and having a longitudinal axis extending in vertical direction. The sensing projection is much smaller than the body and has strain gages affixed on its side surface. A test piece is placed on the distal (upper) end of the sensing projection. When a downward impact load is applied to the test piece from a hammer or striker, the impact load is transmitted to the distal end of the sensing projection to produce a stress wave there. The stress wave produced at the distal end of the sensing projection propagates to its proximal end and thence into the body of the stress sensing block.
The propagating stress wave produces a corresponding dynamic strain of the sensing projection, which is sensed by means of the strain gages affixed on the side surface of the projection. The outputs of the strain gages are processed to determine the dynamic stress produced in the sensing projection, from which the impact load applied to the test piece is obtained. Because only a small portion of the energy of the stress wave propagated into the body may enter again into the sensing projection to return to the strain gages, the impact load measurement is not severely affected by the reflections of the stress wave even when an impact load of relatively long duration is applied to the test piece.
By using the stress sensing block in place of a long Hopkinson bar, a more compact material testing machine may be designed, so as to eliminate the shortcomings of the Hopkinson bar method described above. The article also reports that the stress sensing block may provide good measurement accuracy.
While a material testing machine using such stress sensing block may provide good results in impact compression tests, it is still subject to certain drawbacks in impact tensile tests, in which an impact tensile load is applied to the test piece to produce a tensile strain of the test piece at a high strain rate. That is, the stress sensing block described in the article requires complicated test piece attachments in order to conduct a tensile test. It is difficult to ensure a sufficiently long stroke of the hammer or striker for striking the test piece installed on the stress sensing block so as to apply an impact tensile load to the test piece. Further, the stress sensing block does not allow the use of such test piece attachments that may effectively preventing noise and disturbance to the load measurements.
In view of the foregoing, it is an object of the present invention to provide a material testing machine which is usable for material tests conducted at different strain rates in a wide strain-rate range covering from relatively low strain rates to relatively high strain rates, as well as for material tests necessitating relatively large deformation of the test piece. The material testing machine should also provide load measurements that contain only a low level of noise even in a high-strain-rate material test conducted at a strain rate of 103/sec. or higher, as well as provide precision measurements throughout the duration of an impact load including the initial phase of the impact load, in which a high level of noise is likely to occur with conventional material testing machines. The material testing machine should be also usable for material tests using various test pieces differing in geometry, such as of a circular-rod-type and a flat-strip-type. The material testing machine should be also usable, with or without a test piece attachment or a set of test piece attachments if appropriate, for a variety of material tests including compression test, tensile test, shearing test, fracture toughness test and others, which may be conducted at different strain rates in a wide strain-rate range.
It is another object of the present invention to provide a test piece attachment set usable with such a material testing machine so as to improve flexibility of the machine.
It is a further object of the present invention to provide a method of conducting a material test with such a material testing machine.
In accordance with an aspect of the present invention, there is provided a material testing machine having a frame, load applying means for applying a load in a predetermined direction to a test piece and load measuring means for sensing a load applied to the test piece.
The load measuring means comprises a load sensing block having a body with sufficient volume and mass and at least one sensing projection. The sensing projection is sufficiently smaller than the body of the load sensing block. The sensing projection has a distal end, a proximal end connected to the body of the load sensing block, a longitudinal axis extending in the predetermined direction and a side surface.
The load measuring means further comprises a plurality of strain gages affixed on the side surface of the sensing projection and processing means for processing outputs of the strain gages to determine a load acting on the sensing projection.
The load sensing block is arranged such that a stress wave produced in the sensing projection by an impact acting on the distal end of the sensing projection may propagate along the longitudinal axis of the sensing projection from the distal end to the proximal end and that a first part of the energy of the stress wave reaching the proximal end may further propagate into the body of the load sensing block to reach peripheral surfaces of the body and reflect again and again from one peripheral surface to another so that the stress wave in the body will finally decades out to lose dynamic behavior thereof.
The load sensing block is arranged such that a second part of the energy of the stress wave reaching the proximal end may be reflected at the proximal end to return back to the distal end to create shuttling echoes of the stress wave between the distal and proximal ends and that the sensing projection has a sufficiently short length so that the shuttling echoes may have a turnaround time sufficiently shorter than the duration of the impact applied to the distal end so as to prevent dynamic behavior of the sensing projection due to the stress wave from substantially effecting on the measurement provided by the load measuring means.
The load measuring means is capable of measurement of static and quasi-static loads with accuracy by using the strain gages to sense any static and quasi-static strains of the sensing projection produced by static and quasi-static loads applied to the distal end of the sensing projection and of measurement of impact loads with accuracy by using the strain gages to sense any dynamic strains of the sensing projection produced by dynamic loads applied to the distal end of the sensing projection.
The load applying means comprises a load applying block, guide means for guiding the load applying block for movement in the predetermined direction, drive means for driving the load applying block in the predetermined direction and control means for controlling the driving means.
Finally, the material testing machine is capable of installation of the test piece thereto such that any loads applied by the load applying block to the test piece may be transmitted to the distal end of the sensing projection.
The load applying block may have a body with sufficient volume and mass and at least one load applying projection projecting from the body of the load applying block. Further, the load applying block may be arranged such that a stress wave produced in the load applying projection by an impact acting thereon may propagate into the body of the load applying block to reach peripheral surfaces of the body and reflect again and again from one peripheral surface to another, so as to prevent dynamic behavior of the load applying block due to the stress wave from substantially effecting on a load applied to the test piece.
In one embodiment of the material testing machine, the predetermined direction is vertical direction and the load applying block is disposed above the load sensing block. The load sensing block has a top surface facing the load applying block and having a pair of the sensing projections formed thereon. The load sensing block has a receptacle formed in the top surface between the pair of sensing projections, for receiving the test piece having test piece attachments connected thereto. Finally, the pair of sensing projections are capable of placement thereon of a first test piece attachment connected to the test piece for installation of the test piece to the material testing machine.
In another embodiment of the material testing machine, the load sensing block has a side ridge protruding in transverse direction with respect to the predetermined direction from the body of the load sensing block. The side ridge has an end surface extending in transverse direction with respect to the predetermined direction. The load sensing block has the sensing projection provided on the end surface. Finally, the sensing projection has a connecting portion at the distal end thereof for connection with the test piece.
In accordance with another aspect of the present invention, there is provided a test piece attachment set used for installation of a test piece to a material testing machine as mentioned above, for conducting a tensile test. The test piece has first and second ends to be directed toward the load applying block and the load sensing block, respectively, when the test piece is installed to the material testing machine. The test piece attachment set comprises first and second test piece attachments for connection to the first and second end of the test piece, respectively. The first test piece attachment being adapted for placement on the distal ends of the pair of sensing projections while connected to the first end of the test piece. The second test piece attachment being adapted for engagement with the load applying block while connected to the second end of the test piece. Finally, application of a compressive load by the load applying block to the second test piece attachment results in application of a tensile load between the first and second ends of the test piece.